The invention relates to a multi-frequency dielectric resonator oscillator, and more particularly to oscillators operating at microwave frequencies.
Microwave oscillators are used in transmission systems and more particularly close to the antenna in order to carry out a frequency transposition between an intermediate frequency band and a transmission frequency band. The frequency transposition is due to the use of two units, one generally located inside a building while the other is located outside, close to the antenna. The internal and external units are linked by means of an electrical conductor, for example a coaxial cable, which leads to high losses at the transmission frequencies used, for example greater than 10 GHz.
Communication systems use increasingly high transmission frequencies and increasingly wide frequency bands. A single frequency transposition involves using a band which is as wide for the intermediate frequencies as for the transmission frequency. However, the use of an intermediate frequency bandwidth greater than a GHz has implementation problems. This is because the losses in the coaxial cable which connects the internal and external units vary significantly over the intermediate bandwidth. Moreover, it is expensive to produce a frequency synthesizer which is able to provide the agility in terms of frequency needed to scan very wide intermediate frequency bands.
In order to obtain an intermediate frequency band which is narrower than the transmission band, it is possible to segment the transmission band before transposing it. Such a segmentation may be carried out by using several switched oscillators in the external unit. Depending on the oscillator selected, the intermediate frequency band receives only part of the transmission band, the said part corresponding to the frequency selected.
Dielectric resonator oscillators are commonly used in external transmission system units. Dielectric resonators are generally used in a feedback loop, for example as shown in one of FIG. 1A or 1B. The coupling between the dielectric resonator DR and the electrical circuit is produced by bringing a dielectric resonator DR and conductor CL of the electrical circuit close together, the conductor being, for example, a microstrip line. The positioning of the dielectric resonator will have a direct effect on the degree of coupling, the output power of the oscillator, the frequency stability and also the oscillator frequency.
The oscillation frequency of a dielectric resonator oscillator depends on its dimensions and on the electromagnetic properties of its environment. To control these factors, the resonator is placed in a screened cavity through which the conductor passes. FIG. 2 illustrates an embodiment of this sort in which a microstrip line CL passes through a screened cavity CAV inside which there is a dielectric resonator DR. The resonator DR is held in the cavity CAV by spacers (not shown). For further details on producing dielectric resonator oscillators, a person skilled in the art may refer to the book entitled xe2x80x9cDielectric Resonatorsxe2x80x9d, 2nd edition, by Darko Kajfez and Pierre Guillon, published in 1998 by Noble Publishing Corporation.
A known solution for obtaining several frequencies from dielectric resonator oscillators is to use several oscillators and to switch their outputs. The increase in the number of oscillators leads to an increase in the size of the circuits placed in the external unit.
The aim of the invention is to reduce the size of external units using several transposition frequencies. For this, the invention provides a dielectric resonator oscillator whose oscillation frequency may take several values. The device of the invention uses switched interfering conductors close to the dielectric resonator in order to vary the frequency of the oscillator while using a single resonator, the interfering conductors behaving either as two separate conductors or as a metal plane.
The invention is an oscillator circuit comprising a dielectric resonator located close to a coupling conductor providing the coupling with the rest of the oscillator circuit, the said resonator and the said coupling conductor being located in a cavity. The oscillating circuit comprises at least one pair of interfering conductors located in the cavity close to the resonator. Switching means may or may not establish an electrical contact between the interfering conductors.
The invention will be better understood, and other particular features and advantages will become apparent on reading the following description, the description referring to the appended drawings among which: